A total defection from the Christian religion, after previous acceptance through faith and baptism. Refusal to accept a particular tenet of the faith is properly called heresy.

Apostasy may be merely interior, or exteriorly manifested as well. It may be formal (with full consciousness of the obligation to remain in the faith), or material (without such consciousness). Exterior formal apostasy involves excommunication, reserved in a special manner to the Holy See.

Apostasy from religious life is the unauthorized departure from a religious house of an inmate under perpetual vows, with the intention of not returning; or, if the departure be legitimate, a subsequent refusal to return in order thus to withdraw from the obligations of religious obedience. Such apostates incur excommunication.